1. Field of the Invention
This invention is directed to a method and apparatus for duplex communication in very large mixing networks or intercoms. More particularly, the invention is directed to distributed processing of audio data between multiple audio stations in a large mixing network.
2. Description of Prior Art
Central, conference controllers in telephone networks are well known. In these networks, each telephone is connected to a central processing station. The processor at that central phone station receives the audio signals to be combined in a conference connection, mixes the signals and sends the mixed audio signal to the telephones in the conference connection. Some examples of conference controllers using a central processing station are U.S. Pat. Nos. 4,224,688, 4,271,502 and 5,027,347.
In addition, U.S. Pat. No. 4,646,288 entitled, "Multi-Line Accumulator/Multiplexer" and issued to J. L. Shumway shows an example of a digital implementation of a multiplexer that could be used at a central control station for conferencing. Shumway sums digital audio from time division multiplex channels in an accumulator. He uses a buffer that toggles between being connected to the TDM bus and the accumulator. The accumulated audio signal is then sent out. The accumulator/multiplexer is a one-way system so the central station requires two such devices for duplex operations.
U.S. Pat. No. 4,575,845, entitled "Time Division Multiplex Conferencer" and issued to L. Baranyai et al, describes a one-way digital audio mixing system for a central station. Baranyai et al performs the same task as Shumway, but Baranyai has no storage of digital audio data prior to the summer or accumulator. Accordingly, there is only one opportunity per TDM time frame to sum time slots in the frame into mixed, or conference, audio data. Since the flow is one way through the device, two such devices are required at the central station for duplex conferencing.
All of the prior art devices are performance limited by their use of central digital circuits for conferencing the digital audio signals. In addition, they are directed to conferencing rather than mixing. In conferencing, all audio signals would have the same gain, while in mixing, each audio signal would have separate selectable gain. Accordingly, the conference network prior art is not designed to handle mixing, and could not handle a large mixing network without quickly loading down the central processor and running out of processing power.